Industrial robots are well known in the art. Such robots are intended to replace human workers in a variety of assembly tasks. It has been recognized that in order for such robots to effectively replace human workers in increasingly more delicate and detailed tasks, it will be necessary to provide sensory apparatus for the robots which is functionally equivalent to the various senses with which human workers are naturally endowed, e.g. sight, hearing, etc.
Of particular importance for delicate and detailed assembly tasks is the sense of touch. Touch can be important for close-up assembly work where vision may be obscured by arms or other objects, and touch can be important for providing the sensory feedback necessary for grasping delicate objects firmly without causing damage to them. Touch can also provide a useful means for discriminating between objects having different sizes, shapes or weights. Accordingly, various tactile sensors have been developed for use with industrial robots.
One such tactile sensor, developed by William D. Hillis and John Hollerback, is disclosed in the article, "How Smart Robots Are Becoming Smarter", by Paul Kunnucan, High Technology Magazine, Sept./Oct. issue, pp. 32,36, Technology Publishing Company. This tactile sensor comprises 256 pressure-sensitive electrical switches arranged in a 16.times.16 grid pattern. When an object is brought into contact with the sensor, appropriate switches are triggered so as to produce a pattern of electrical signals which correspond to the "feel" of the object contacting the sensor. Hillis and Hollerback constructed their tactile sensor by sandwiching a piece of ordinary pantyhose between a top layer comprised of a piece of non-conductive silicon rubber impregnated with conductive graphite along 16 parallel lines and a bottom layer comprised of a printed circuit board having 16 parallel conducting lines disposed therein. Hillis and Hollerback oriented their top and bottom layers relative to one another so that the 16 parallel conducting lines in the top layer were disposed at right angles to the 16 parallel conducting lines in the bottom layer, thereby forming a sensory array comprising 256 superimposed intersection points arranged in a 16.times.16 grid pattern. The conducting lines of the top layer are normally separated from the conducting lines of the bottom layer by the pantyhose. However, when an object comes into contact with the sensor and exerts a requisite minimum pressure on the sensor, the conducting lines make contact with one another through the meshes of the pantyhose at those intersection points disposed beneath the points of pressure, thereby allowing current to flow between the top and bottom layers at selected locations so as to produce a pattern of electrical signals which corresponds to the "feel" of the object contacting the sensor.
This same concept is believed disclosed in another publication, "Active Touch Sensing" by William Daniel Hillis, distributed by the MIT Artificial Intelligence Laboratory as A.I. Memo 629. In this second publication, Hillis also describes replacing the intermediate layer of pantyhose with an alternative separator layer comprised of non-conductive paint. This non-conductive paint is sprayed directly onto the bottom side of the top layer as a fine mist so that it adheres thereto as a collection of spaced, non-conductive dots. The conducting lines of the top layer are normally separated from the conducting lines of the bottom layer by the misted layer of non-conductive paint. When an object comes into contact with the sensor and exerts a requisite minimum pressure on the sensor, the conducting lines of the top layer make contact with the conducting lines of the bottom layer through the gaps existing between adjacent dots of non-conductive paint at those intersection points disposed beneath the points of pressure, thereby allowing current to flow between the top and bottom layers at selected locations so as to produce a pattern of electrical signals which corresponds to the "feel" of the object contacting the sensor.
U.S. Pat. No. 4,208,648 discloses another tactile sensor which comprises 256 pressure-sensitive electrical switches arranged in a 16.times.16 grid pattern, such that when an object is brought into contact with the sensor, appropriate switches are triggered so as to produce a pattern of electrical signals corresponding to the "feel" of the object contacting the sensor. The tactile sensor disclosed in U.S. Pat. No. 4,208,648 differs significantly from the aforementioned sensors in many of the particulars of its construction and operation. More particularly, the sensor disclosed in U.S. Pat. No. 4,208,648 comprises three distinct layers disposed in a sandwich arrangement. The top layer is comprised of a resilient, electrically-insulating material in which is disposed a first set of 16 parallel conducting lines. The bottom layer is comprised of a resilient, electrically-insulating material in which is disposed a second set of 16 parallel conducting lines. The bottom layer is disposed relative to the top layer so that the second set of conducting lines extend at right angles to the first set of conducting lines, thus forming a sensory array comprising 256 superimposed intersection points arranged in a 16.times.16 grid pattern. The intermediate layer is comprised of a synthetic resin material which is electrically conductive to some extent in its natural, unstressed state, and whose conductivity is increased by compression. When an object exerts a requisite minimum pressure on the sensor, the conductivity of the intermediate layer is altered at those locations disposed beneath the points of pressure. As a result, the current flowing between the first and second sets of conducting lines is also altered at the locations affected by pressure, and a pattern of electrical signals is produced which corresponds to the "feel" of the object contacting the sensor.
Unfortunately, the tactile sensors described above are believed to suffer from one or more of the following limitations: (1) poor reliability, (2) poor durability, (3) high cost of manufacture, (4) significant complexity of manufacture, (5) limited sensitivity, (6) complicating signal "cross-talk" (where the current signal travels through the intermediate layer at locations other than those disposed at the points of pressure) and (7) a pressure/current relationship of limited utility.